Synthesis of palladium nanoparticles stabilized on Schiff base-modified ZnO particles as a nanoscale catalyst for the phosphine-free Heck coupling reaction and 4-nitrophenol reduction

Recently, the development of heterogeneous nanocatalytic systems using solid supports has been gaining importance due to some advantages such as easy handling, high thermal stability, high efficiency, reusability, and so on. Therefore, the design of catalyst supports for the preparation of stable heterogeneous catalytic systems is of great importance. In this work, Schiff base-modified ZnO particles have been developed (ZnO–Scb) as a novel support. A heterogeneous nanocatalyst system has then been prepared by immobilizing palladium nanoparticles (Pd NPs) on the ZnO-Scb surface as the support. The resulting palladium nanocatalyst (Pd–ZnO–Scb) structure has been characterized by different analytical techniques (FT-IR, XRD, TEM, FE-SEM, elemental mapping and EDS) and used to catalyze the Heck coupling reactions and 4-nitrophenol (4-NP) reduction. Test results revealed that Pd–ZnO–Scb could effectively couple various aryl halides with styrene in yields of up to 98% in short reaction times. Pd–ZnO–Scb was also efficiently used in the complete 4-NP reduction within 135 s at room temperature. Additionally, it was found that Pd–ZnO–Scb was more effective than other reported catalysts in the Heck coupling reaction. Moreover, the recycling tests indicated that Pd–ZnO–Scb could be easily isolated from the reaction medium and reused in seven consecutive catalytic runs while retaining its nanostructure.

Pd-ZnO-Scb-catalyzed 4-NP reduction. The mixture of an aqueous solution of 4-NP (2 mL, 1.5 × 10 -4 M) and NaBH 4 (0.5 mL, 0.05 M) was stirred for 2 min at room temperature. 10 mg of Pd-ZnO-Scb were then transferred into the media to start the catalytic reduction of 4-NP and the resulting mixture was stirred for the desired time. The progress of 4-NP reduction was followed by UV-Vis spectroscopy.

Results and discussion
Pd-ZnO-Scb characterization. The formation of ZnO, ZnO-NH 2 , and ZnO-Scb were confirmed by FT-IR analysis and the corresponding spectra are given in Fig. 1. Generally, ZnO displays an absorption band below 600 cm −1 related to the Zn-O stretching vibration 45 . However, this stretching band could not be detected due to the range of ATR/FT-IR. Therefore, XRD, EDS, and FE-SEM analyses were performed to study the fabrication of ZnO. Figure 1b shows the spectrum of ZnO-NH 2 . The strong peaks located at 2926 and 2867 cm −1 were assigned to the aliphatic C-H stretching vibrations of APTES molecules. Additionally, the peaks at 1575 and 1012 cm −1 were attributed to the NH 2 deformation and Si-O stretching vibrations. These important and characteristic peaks confirmed the successful attachment of APTES to the surface of ZnO 46 . Additionally, a ninhydrin color test was performed to confirm the presence of APTES molecules on ZnO surface. For this purpose, 5 mg of ZnO-NH 2 and 5 mg of ninhydrin were refluxed in 10 mL of ethanol 47 . After 3 min, the color of the suspension turned to purple, resulting in an absorbance peak of 580 nm in the UV-Vis spectrum ( Figure S1). Based on the color test and UV-Vis analysis, APTES molecules are attached to the surface of ZnO. The peak at 1645 cm −1 in the FT-IR spectrum of ZnO-Scb indicated the presence of imine stretching vibration, confirming that the condensation reaction of ZnO-NH 2  www.nature.com/scientificreports/ respectively 48 . These results indicated the high purity of the prepared ZnO. Following the preparation of Pd NPs, ZnO peaks did not change while new peaks were observed at 40.16°, 46.60°, and 82.23°, which were associated with the (111), (200), and (220) planes of Pd, respectively, confirming the stabilization of Pd NPs on the support 49 .
To examine the morphological properties of synthesized ZnO, ZnO-NH 2 , ZnO-Scb, and Pd-ZnO-Scb, FE-SEM analysis was performed and the images are illustrated in Figure S2. As it can be observed in Figures S2a-c, ZnO NPs were aggregated and spherical. After the fabrication of ZnO-NH 2 and ZnO-Scb, it was observed that ZnO surface was covered with particles, confirming the successful chemical modification of ZnO. The FE-SEM images of Pd-ZnO-Scb indicated the deposition of Pd NPs on ZnO-Scb surface with a nearly spherical shape. EDS analyses were performed to determine the presence of elements in ZnO, ZnO-NH 2 , and Pd-ZnO-Scb. Additionally, elemental mapping analysis was conducted to investigate the distribution of elements on the surface of the Pd-ZnO-Scb complex. The corresponding spectra are displayed in Figures S3 and S4, respectively. As observed in Figure S3, the EDS spectrum of ZnO showed peaks corresponding to Zn and O elements. The EDS spectrum of ZnO-NH 2 displayed the presence of C, N, and Si peaks, related to APTES molecules, which confirmed that APTES molecules were attached to the surface of ZnO. As for Pd-ZnO-Scb spectrum, Pd peaks were observed in addition to the expected elements such as C, N, O, Si, and Zn. Additionally, the presence of Pd was determined using elemental mapping ( Figure S4); which indicated the uniform dispersion of Pd on the ZnO-Scb surface.
To further examine the shape and size of Pd-ZnO-Scb, TEM analysis was carried out (Fig. 3). The TEM images of Pd-ZnO-Scb clearly indicated the homogeneous dispersion of the Pd NPs, observed as spherical black spots, on its support. The average diameter of Pd NPs was about 14 nm (Fig. 4).

Investigation of catalytic activity of Pd-ZnO-Scb.
After complete characterization of Pd-ZnO-Scb, its catalytic prowess was evaluated in the Heck coupling reaction. In the early step, the reaction between styrene and 1-bromo-4-nitrobenzene was selected as a model reaction and the effects of time, solvent, base, and catalyst loading were then studied to determine the optimal conditions. As observed in Fig. 5, the optimal reaction conditions were 15 mg of Pd-ZnO-Scb, temperature of 120 °C, Na 2 CO 3 as the base, and DMF as the solvent. Afterward, the generality and substrate tolerance of Pd-ZnO-Scb catalytic system was tested in the Heck coupling reaction of different substituted aryl halides under optimal reaction conditions and the results are shown in Table 1. The reaction of styrene and aryl iodides bearing different groups such as -OMe, -Me, and -NO 2 was successfully performed with good reaction yields. For example, 4-iodoanisole was converted to the target product with 95% yield within 1.5 h. 1-Iodo-3-nitrobenzene was coupled with styrene with 92% yield. The catalytic potential of Pd-ZnO-Scb was also tested using different substituted aryl bromides and it was found that Pd-ZnO-Scb successfully catalyzed these reactions by providing good isolated yields. For example, 4-bromotoluene was reacted with styrene and the corresponding Heck product reached 91% yield within 3 h. Bromobenzene substrate formed the corresponding Heck product with 94% yield. The reaction of 4-bromobenzonitrile gave the desired product in 96% isolated yield in 1.5 h. The results obtained clearly showed that Pd-ZnO-Scb played a crucial role in the Heck coupling reactions. On the other hand, the catalytic performance of some previously reported catalysts in the Heck coupling reaction between 4-iodoanisole and styrene have been summarized to compare with that of our catalyst ( Table 2). It is evident that Pd-ZnO-Scb performed better than the other catalysts in terms of reaction yield and time.
Inspired by the performance of our catalyst in the Heck coupling reaction, it was also used in the catalytic reduction of 4-NP to 4-AP by NaBH 4 . The reduction of 4-NP in the presence of a catalyst is simple compared  www.nature.com/scientificreports/ to other methods due to the formation of merely one product (4-AP). In addition, the reaction progress can be easily followed by UV-Vis spectroscopy at 400 nm 60,61 . As Fig. 6a displays, 4-NP, which has a pale-yellow color, showed a maximum absorption at 317 nm in water solvent. Upon the addition of freshly prepared NaBH 4 solution into 4-NP solution, the color of the solution changed to deep yellow and the absorption peak at 317 nm shifted to about 400 nm, indicating the formation of 4-nitrophenolate anion. Therefore, 4-NP reduction is carried out on 4-nitrophenolate anion (4-NPT). It was observed that this solution was very stable and the absorption peak at 400 nm did not change even after 5 h without Pd-ZnO-Scb. This indicated that Pd-ZnO-Scb was required for 4-NP reduction. Upon the addition of Pd-ZnO-Scb into 4-NP + NaBH 4 mixture, the absorption at 400 nm gradually decreased and disappeared within 135 s. Additionally, a new peak appeared at about 300 nm simultaneously with the reduction reaction, confirming the formation of 4-AP. Moreover, the yellow color of the reaction solution turned colorless at the end of the catalytic reduction. All these findings confirmed the successful conversion of 4-NP to 4-AP by Pd-ZnO-Scb within 135 s without any side products, in concordance with previous studies.  The rate constant was found as 0.007 s −1 for 4-NP reduction using the following equation. The catalytic system followed pseudo-first-order kinetics due to NaBH 4 concentration being higher than 4-NP.  where c 0 and c are the initial and final concentrations of 4-NP at tested reaction time (t), respectively, and k (s −1 ) is the reaction rate. Table 3 summarizes the comparison of the catalytic prowess of Pd-ZnO-Scb with those of various other catalysts in 4-NP reduction. The results showed that Pd-ZnO-Scb was the most active among the catalysts.

Recyclability potential of Pd-ZnO-Scb.
It is known that the recyclability of a catalyst is a crucial factor for both industrial and academic applications in terms of economy, cost-effectiveness, labor, and sustainability. Therefore, the recycling potential of Pd-ZnO-Scb nanocatalyst was investigated in the model Heck coupling reaction under optimized conditions. After each cycle, Pd-ZnO-Scb was isolated by filtration, rinsed with water and ethanol, and dried. The recovered Pd-ZnO-Scb was then directly used in the next reactions. The results revealed that Pd-ZnO-Scb could be reused and recycled up to 7 successive runs giving a product yield of 87%.
To check the stability of Pd-ZnO-Scb, its surface was examined by TEM and XRD analyses following the seventh run and the corresponding images are given in Figures S5 and S6. It was found that the particle size, shape, and morphology of the recycled Pd-ZnO-Scb were almost identical to that of the fresh catalyst, indicating the structural stability of Pd-ZnO-Scb.
A hot filtration test was performed on 1-bromo-4-nitrobenzene under the optimal reaction conditions determined in the Heck coupling reaction. The Heck coupling reaction was carried out for 45 min in the presence of Pd-ZnO-Scb nanocatalyst. The catalyst was then recovered from the reaction medium and allowed to react for an additional 45 min under the same conditions to complete the reaction time with the filtrate. It was observed that the Heck coupling reaction did not progress further, indicating that Pd-ZnO-Scb did not leach. Inductively Coupled Plasma (ICP) analysis revealed that the content of Pd NPs loaded on the ZnO-Scb surface was about 9.6%. To check the heterogeneity of catalyst, which is an important factor, the phenomenon of leaching was studied by ICP analysis of the resulting reaction mixture. According to the ICP analysis, the Pd content of the used catalyst was determined as 9.2%. ln (c/c 0 ) = −kt

Conclusion
In summary, Pd-ZnO-Scb has been designed as a retrievable/recyclable heterogeneous nanocatalyst by stabilizing Pd NPs on the prepared Schiff base functionalized ZnO support. The nano-structured Pd-ZnO-Scb was successfully characterized by FT-IR, FE-SEM, TEM, XRD, elemental mapping, and EDS analyses. The catalytic prowess of Pd-ZnO-Scb was then evaluated in the Heck coupling reaction and 4-NP reduction. The results indicated that Pd-ZnO-Scb successfully coupled aryl chlorides, bromides and iodides with styrene, giving 63-98% isolated yields. 4-NP reduction was also efficiently catalyzed by Pd-ZnO-Scb in a short reaction time (135 s). Furthermore, Pd-ZnO-Scb was utilized for seven successive runs with 87% reaction yield and the protection of the nanostructure following the recycling tests was confirmed by TEM analysis. Due to its low cost, simplicity of separation, high yield, stability, and high recoverability/reusability, Pd-ZnO-Scb has a high potential for catalytic transformations and therefore, its catalytic prowess can be evaluated in other applications in the future.

Data availability
All data generated or analyzed during this study are included in this published article and its supplementary information files.